A variety of applications involve transmitting charged particles from one location to another. This is the case, for example, in the transmission of ions to a quadrupole mass filter and other types of mass spectrometers, and in the pulsed release of trapped ions from a multipole ion guide ion trap into the pulsing region of a time-of-flight (TOF) mass spectrometer. In addition, there is a need for an improved and finer focusing of charged particle beams in general. However, the transmission of selected ions is often accompanied by the concomitant transmission of undesirable particles, which may include charged particles with substantially higher and/or lower kinetic energy, as well as uncharged particles such as neutral species and photons.
These undesirable particles can interfere with the transmission or optimum utilization of the desired particles. For example, energetic charged and uncharged particles, such as aerosols, frequently accompany the introduction of analyte ions from an atmospheric pressure ion source into vacuum, and such ‘background’ particles may be very energetic, making them difficult to control or eliminate. Other examples of undesirable ‘background’ particles include photons that are produced, for example, in inductively coupled plasma (ICP) ion sources and electron induced ionization (EI) ion sources, as well as neutral metastable particles such as are produced in such ion sources, which can transfer energy to secondary particles to create unwanted secondary ions. All such particles can lead to undesirable consequences, such as background noise at the detector of a mass spectrometer.
In other scenarios, such as the technique of secondary ion mass spectrometry (SIMS), a finely-focused energetic ion beam impacts a small surface area of a solid sample, thereby releasing so-called secondary ions of sample surface species. Subsequent mass analysis of these secondary ions reveals the chemical composition of this small area of sample surface. However, energetic neutral particles accompanying the focused ions are not focused, and their impact creates sample surface secondary ions from outside the analysis area, which degrades the spatial specificity of the analysis.
Further, kinetic energy filters and analyzers are configured to pass charged particles with a relatively narrow range of energies with high transmission, while preventing as many particles as possible with kinetic energies outside this narrow range from passing to a detector.